extruding circular shape
To scale in one plane, use Shift+ the axis you don't want to scale in.

For example, if you want to extrude and scale in the YZ plane, you would do E&rarr;S&rarr;Shift+X.

If you use the 3D Manipulator, Shift+clicking the handle of the axis you don't want to scale (e.g. Shift+click the X-axis handle for the YZ plane).